MUMBAI, India – Five NATO troops died in a helicopter crash in southern Afghanistan on Saturday in the deadliest incident so far this year for the international forces, officials said.

The U.S.-led NATO coalition said it was investigating the incident and did not offer additional details. The nationalities of the soldiers weren’t released.

“Our thoughts and prayers go out to the family and friends affected by this tragic event,” the coalition said in a statement.

Deaths among international troops have declined sharply in Afghanistan as NATO transfers security responsibilities to Afghan soldiers and police and prepares to withdraw most of its forces by the end of the year. But there has been a sharp uptick in violence aimed at Western civilians, with 22 killed since January, the most recent being three Americans shot dead at a Kabul hospital on Thursday.

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Two of the Americans were named Saturday as Jon Gabel, who ran a health clinic at Kabul University, and his father Gary Gabel. They were identified by a university official, Mohammad Hadi Hadayati. A pediatrician from the Chicago area, Jerry Umanos, also was killed.

Last December, six American troops died in a helicopter crash in the southern province of Zabul. NBC News later reported that military officials determined the aircraft was brought down by an enemy attack, likely from the Taliban.